Datasheet

NCV8873
http://onsemi.com
10
The maximum average inductor current can be calculated
as follows:
I
L,avg
+
V
OUT
I
OUT(max)
V
IN(min)
The peak inductor current can be calculated as follows:
I
L,peak
+
V
IN(min)
D
max
Lf
s
Where: I
L,peak
: Peak inductor current value [A]
4. Select Output Capacitor
The output capacitor smoothes the output voltage and
reduces the overshoot and undershoot associated with line
transients. The steady state output ripple associated with the
output capacitors can be calculated as follows:
V
OUT(ripple)
+
I
LED
ǒ
1 * d (M
max
)
Ǔ
f
s
C
OUT
The capacitors must withstand an RMS ripple current as
follows:
I
Cout(RMS)
+ I
LED
2
) d(M
max
)
ǒ
I
L,pk
2
3
* I
L,pk
I
LED
Ǔ
Ǹ
A 2.2 mF ceramic capacitor is usually sufficient for high
brightness LED applications for f
s
= 1 MHz.
5. Select Input Capacitors
The input capacitor reduces voltage ripple on the input to
the module associated with the ac component of the input
current.
I
Cin(RMS)
+
ǒ
D(M
max
) ) d (M
max
)
3
Ǔ
I
L,pk
2
* I
L,avg
2
Ǹ
6. Select Feedback Resistors
The feedback resistor provides LED current sensing for
the feedback signal. It may be calculated as follows:
R
F1
+
V
ref
I
LED
7. Select Compensator Components
Current Mode control method employed by the NCV8873
allows the use of a simple Type II compensation to optimize
the dynamic response according to system requirements. A
transconductance amplifier is used, so compensation
components must
be connected between the compensation
pin and ground.
8. Select MOSFET(s)
In order to ensure the gate drive voltage does not drop out,
the selected MOSFET must not violate the following
inequality:
Q
g(total)
v
I
drv
f
s
Where: Q
g(total)
: Total Gate Charge of MOSFET(s) [C]
I
drv
: Drive voltage current [A]
f
s
: Switching Frequency [Hz]
The maximum RMS Current can be calculated as follows:
I
Q(max)
+ I
L,peak
D(M
max
)
3
Ǹ
The maximum voltage across the MOSFET will be the
maximum output voltage, which is the higher of the
maximum input voltage and the regulated output voltage:
V
Q(max)
+ V
OUT(max)
9. Select Diode
The output diode rectifies the output current. The average
current through diode will be equal to the output current:
I
D(avg)
+ I
OUT(max)
Additionally, the diode must block voltage equal to the
higher of the output voltage or the maximum input voltage:
V
D(max)
+ V
OUT
The maximum power dissipation in the diode can be
calculated as follows:
P
D
+ V
f (max)
I
OUT(max)
Where: P
d
: Power dissipation in the diode [W]
V
f(max)
: Maximum forward voltage of the diode
[V]
Low Voltage Operation
If the input voltage drops below the UVLO or MOSFET
threshold voltage, another voltage may be used to power the
device. Simply connect the voltage you would like to boost
to the inductor and connect the stable voltage to the VIN pin
of the device. In Boost configuration, the output of the
converter can be used to power the device. In some cases it
may be desirable to connect 2 sources to VIN pin, which can
be accomplished simply by connecting each of the sources
through a diode to the VIN pin.